TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL ACTIVATION OF GENES IN SALT-TOLERANT ALFALFA CELLS

Salt-tolerant cell lines of alfalfa (Medicago sativa L.) selected in t his laboratory showed increased mRNA accumulation for both nuclear- an d chloroplast-encoded genes involved in photosynthesis as well as in s everal non-photosynthetic related functions. The basis for this consti tutive and salt-dependent gene activation was investigated by measurin g both nuclear and plastid run-on transcription from the salt-sensitiv e parent line and from selected salt-tolerant lines. Plastids from tol erant cells showed a 2.5-fold increase in transcription rate over thos e from sensitive cells and a 4.5-fold increase if isolated from tolera nt cells grown in salt. Nuclei isolated from salt-tolerant cells grown on normal medium showed higher transcription of the photosynthesis-re lated genes rbcS, cab1 and cab4 than those from salt-sensitive cells, confirming that the salt-tolerant cells had acquired altered transcrip tional regulation of these genes. However, the major salt-induced incr ease in steady-state mRNA accumulation, from photosynthesis-related an d other genes (alfin1, pA18 and histone H3cI and H3cII genes), was not reflected in run-on assays from these same cells. These results indic ated that salt-dependent post-transcriptional mRNA stabilization led t o the steady-slate mRNA accumulation. The mRNA stabilization appears t o be transcript specific, since transcripts of a constitutively-expres sed gene (Msc27) remained unaffected by growth of the tolerant cells i n 171 mM NaCl.